In case of reading data from a cell array of an SRAM (Static Random Access Memory), the reading speed is determined by the cell positioned farthest from the subject sense amplifier in the cell array and this has been a problem. And even in case of writing, a problem has arisen from whether or not single bit information held in the F/F (Flip-Flop circuit) of the cell positioned farthest from the subject write circuit is inverted as soon as the writing is done.
Those problems are caused not only by the characteristics of the transistors of each of the subject SRAM array cells, but also by the parasitic capacity/resistance of the subject bit line. At this time, the capacity/resistance value is affected significantly by the level of the wiring film pressure/inter-layer film pressure employed in a wafer diffusion process.
Techniques related to such ring oscillators composed of conventional SRAM cells respectively are disclosed by U.S. Pat. No. 7,142,064 A and US 2006/0,097,802 A1.